Device and method for production of visual sensations by optic nerve stimulation

ABSTRACT

A method and device for producing visual sensations within blind persons includes installing a self-sizing spiral nerve cuff electrode about the optic nerve of a blind person then transmitting electrical pulses to the nerve to generate phosphenes within the patient&#39;s visual field.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a utility patent application claiming priority from provisionalU.S. Patent Application Ser. No. 60/094,266 filed Jul. 27, 1998, andalso from provisional U.S. Patent Application Ser. No. 60/100,492, filedSep. 16, 1998.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates generally to a method and device forinducing visual sensations within persons suffering from blindness, and,in particular to a method and device for restoring some visual functionwithin the eyes of blind persons by encircling the optic nerve with aself sizing cuff electrode which is electrically stimulated.

Blindness is an affliction which affects more that 42 million personsworldwide. Many causes of blindness, especially those involvingocclusions or opacity in the optical pathway through the eye, haveyielded to medical treatments and vision can now be restored to a greatextent in a large number of cases. Some diseases of the eye, however,cause blindness by attacking the light sensing retina, causing blindnesswhile the remainder of the optical pathway remains functional. A varietyof retinal diseases, for example, cause vision loss or blindness bydestruction of the choroid, choriocapallaris, and the outer retinallayers. In several retinal degenerative diseases, select populations ofphotoreceptor cells are lost. Specifically, in macular degeneration andretinitis pigmentosa, the retinal photoreceptors degenerate while othercells in the retina as well as the retina's central connections aremaintained; the complex synaptic interconnections at the outer plexiformlayer of the eye that would normally transmit photosignals to the nerveganglions are intact, as are the ganglion axons or bundles which make upthe optic nerve, which normally transmits the visual information to thebrain.

Numerous efforts have been made over the years to develop devices whichcould help to remedy retinal blindness. In the 1930's, Foresterinvestigated the effect of electrically stimulating the exposedoccipital pole of one cerebral hemisphere. He found that when a point atthe extreme occipital pole was stimulated, the patient perceived a smallspot of light directly in front and motionless, which spots are known asphosphenes. Subsequently, in the 1960's Brindley and Lewin thoroughlystudied electrical stimulation of the human occipital cortex. By varyingthe stimulation parameters, the investigators described in detail thelocations of the phosphenes produced relative to the specific region ofthe occipital cortex stimulated. These experiments demonstrated: theconsistent shape and position of phosphenes; that increased stimulationpulse duration made phosphenes brighter; and that there was nodetectable interaction between neighboring electrodes which were asclose as 2.4 mm apart.

In recent years, several alternative procedures have been proposed forthe restoration of vision. One procedure involves the implantation of atissue graft or cells into the host retina. One example of thisprocedure is taught in U.S. Pat. No. 5,817,075.

Another procedure which as been attempted uses electrodes or electricalfields in combination with photosensitive devices to stimulate neuronsor ganglion cells. For example, U.S. Pat. No. 4,628,933 is directed to avisual prosthesis having a close packed array of photosensitive deviceswhich are coupled to a plurality of electrodes that stimulate neurons atthe surface of the retina in a pattern corresponding to the illuminationpattern of the photosensitive array. U.S. Pat. No. 5,109,844 describes aretinal microstimulator for stimulating retinal ganglion cells by theuse of a plurality of electrodes for recognition. U.S. Pat. No.5,147,284 teaches an electromagnetic field radiator and receiver whichproduce an effect of electrostimulation of the optic nerve and retina bya pulsed magnetic field. Finally, U.S. Pat. No. 5,873,901 uses a thinfilm optical detector based on a dielectric capacitor that can beimplanted onto the retina which can produce a signal at the brain downfrom the optic nerve that may be perceived as light.

A great variety of electrodes have been developed for application ofelectrical stimulation. Electrodes intended to stimulate motor nervefibers include electrodes placed on the surface of the skin,percutaneous intramuscular electrodes, surgically implantedintramuscular electrodes, and epimysial electrodes, while electrodesplaced directly on or in peripheral nerve trunks include epineuralelectrodes, penetrating epineural electrodes, wire and siliconintraneural electrodes.

A different hurdle in the development of a visual prosthesis has beenthe development of an electrode suitable for chronic implant which canprovide stimuli to the optic nerve. The present invention overcomesthese and other problems by the use of a self-sizing spiral cuffelectrode around the optic nerve.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodof inducing visual sensation within blind persons by generatingelectrical stimuli to the optic nerve via a self-sizing spiral cuffelectrode.

It is also an object of the present invention to provide a device whichcan be implanted intracranially within a patient.

It is a further object of the present invention to produce localizedvisual sensations in blind persons which can be varied by changing themagnitude of the stimulus.

These and other objects of the present invention are accomplished by theuse of a signal generating means which transmits a series of pulses tothe optic nerve of a blind patient via a self-sizing cuff electrode toactivate the nerve, whereby the patient is able to visualize a series ofphosphenes with broad distribution within the visual field.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphic representation of the device of the presentinvention shown in the installed position in a patient;

FIG. 2 is a cross-sectional view of the head of the patient showing theposition of the device of the present invention;

FIG. 3 is a graphic representation showing the current threshold forperception in terms of pulse duration and train frequency;

FIG. 4 is a graphic representation of the retinotopic arrangement ofphosphenes generated using the present invention; and

FIG. 5 is a graphic representation of the retinotopic organization of apatient using the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIGS. 1 and 2, there is shown a prosthetic device,generally indicated at 10, which embodies the principles of the presentinvention. Device 10 comprises a self-sizing spiral cuff electrode 14having a lead 16. Electrode 14 is preferably constructed according tothe type taught in U.S. Pat. No. 5,505,201 or U.S. Pat. No. 5,324,322,the disclosures of which are incorporated by reference in its entiretyfor all purposes. The nature of electrode 14 allows it to be positionedabout an optic nerve 20 of a patient and permitted to helically curlaround and snugly engage the trunk of optic nerve 20, providing intimateand secure electric contact.

To install electrode 14 in position within the skull 22 of the patient,access is gained to skull 22 and electrode 14 is positioned about opticnerve 20 and then released by a unique spiral nerve cuff electrodeimplantation tool which is described in U.S. patent application Ser. No.08/986,943. This tool allows electrode 14 to be sequentially released inposition about optic nerve 20. Lead 16 from electrode 14 was positionedalong the dura mater up to the inferior part of the skull opening. Aftercrossing the dura mater on the lateral aspect of skull 22, lead 16passes through skull 22 and continues below the skin surface to asubcutaneous connector 24 embedded within skull 22 over the ear 26. Alead 28 from connector 24 passes down the neck and exits the skin belowthe clavicle terminating with an external connector 30. A controller 32is coupled to device 10 at connector 30.

The essence of the method and device will now be illustrated by thefollowing case history.

A 59-year old female patient having retinitis pigmentosa was chronicallyimplanted with a device embodying the present invention comprising aself sizing spiral cuff electrode having four contacts installed aroundthe optic nerve. Stimulation started at the hospital on day 2postsurgery. The patient eventually worked up to a level of twothree-hour stimulation sessions a week. Single pulses and pulse trainswere both used for stimulation. Stimulation was either monopolar using asurface indifferent anode, or bipolar between two contacts within theelectrode cuff. Charge density was always kept below 150 μC/(cm² phase)up to 50 Hz [below 50 μC/(cm² phase) up to 333 Hz], corresponding to acharge per phase of 300 nC/phase (respectively 100 nC/phase) with acontact area of 0.2 mm². Determination of current intensity thresholdsfor generation of a phosphene was always done using the 2 staircaselimit method.

To assess phosphene location, a pointing hemisphere, with a radius of0.45 m was used. The patient's head was maintained in front of thehemispheric surface using a stabilizing frame to support the forehead,chin, and parietal skull, with the right eye positioned at hemispherecenter. The right eye EOG was recorded and eye movements were monitoredusing a TV camera.

When ready for a stimulus, the patient placed her head in a fixedposition constrained by the frame, and reached into the hemisphere toplace her left index finger on the fixation point, which was a polymerdisk at the intersection of the visual axis with the hemisphere. Thepatient was then instructed to “look at” the fixation point with asteady gaze throughout the stimulation test run. The test run wasdelimited by two beep sounds, with the left forefinger still in contactwith the fixation point, as a proprioceptive reference, the evokedphosphene was then indicated, with the right hand fingers, as a shape onthe hemisphere. Phosphene characteristics were recorded, including thefollowing quantities: position dimensions and organization; subjectivebrightness; dot diameter; foreground and background colors; motion; etc.

After day 118 post surgery, some 1,465 phosphenes had been documented.Transverse thresholds reached generally twice or more the correspondingmonopolar thresholds, indirectly confirming proper electrode position.There has been no threshold increase over the time since implantation,and electrical stimuli applied to the contacts in the self-sizing spiralcuff electrode have never evoked sensations other than visual.

Most phosphenes were reported to consist of a set of dots, either in acluster of 2 to 5, or else arranged in rows, arrays or lumps of 6 to 30.Dot diameter ranged from 8 to 42 minutes of arc (1 to 5.5 mm at thedistance of 0.45 m). Occasionally, a kind of surround of lesserbrightness was described around each dot in a phosphene. Solid lines,bars, or triangles devoid of dot structure were sometimes reported,usually near perception threshold. Phosphene area (or envelope area fordot phosphenes) generally ranged from 1 to 50 square degrees. Brightnesswas graded on a scale of 1 to 9 with “very, very weak” graded as 1 and“very, very bright” graded as 9.

Phosphenes were often reported as colored. In the first dayspostsurgery, they generally appeared to the patient as gold-yellowagainst a black visual field. Thereafter, blue, white or plain yellowcolors were described. with dot phosphenes, the otherwise black visualfield sometimes appeared colored in blue, red, or yellow, in between thedots. Occasionally, a solid colored surface (red or yellow) wasdescribed adjacent to the envelope of a dot phosphene.

Among the 156 phosphenes collected at the hospital up to day 8 postsurgery, 37% were described as moving; most often, they consisted oflines, instead of dots. Afterward, 1,308 of the next 1,309 phosphenesappeared consistently immobile and steady.

Current intensity thresholds were determined using 5 pulse durations(25, 50, 100, 200, and 400 μs). This study included single pulses, aswell as trains of 5, 9, and 17 pulses generated at 40, 80, and 160 Hzrespectively. As in the classical strength-duration curve, currentintensity thresholds diminished with increasing pulse duration.

FIG. 3 depicts a strength-duration frequency surface for this casehistory. For given values of pulse duration and train frequency, eachdatum point represents the mean current threshold of the four contactsfor perception. By extension, single pulses are referred to as zerofrequency. The representation of the classical strength-duration curve,illustrated here in the zero frequency frontal plane, is also shown fortrains of 100 ms duration. As a result, a clear drop in perceptualthreshold appears along the 5 transverse planes corresponding to thedifferent pulse duration (25, 50, 100, 200, and 400 μs) when thestimulus frequency increases.

It should be observed that, for a given contact in the cuff electrode,attributes of a phosphene, such as perceived brightness, color, size,organization, position, etc., perceived at threshold for a specificpulse duration and train frequency, usually differed from thosedocumented at threshold for another pulse duration and/or trainfrequencies. Furthermore, phosphene attributes reported by the patientdiffered when the same contact was stimulated at different frequenciesbut at the same values of pulse duration and current.

The attributes of the phosphenes were usually consistent for trialsrepeated over a short period of time. For example, when, for a firstphosphene, standard deviations of center of gravity position as afunction of time are 1.1° horizontal and 0.6° vertical (7 measurementsmade on day 84 for 93 min.), for a second phosphene, they reach 2.6°horizontal and 3.2° vertical (9 measurements made on day 81 for 182min., and 8 measurements made on day 84 for 101 min.).

Localized phosphenes were reported to have been perceived over a largerportion of the visual field, up to 35° upwards and 50° downwards on thevertical meridian and 30° leftwards and 30° rightwards on the horizontalmeridian. Near threshold, there appears to be a good retinotopiccorrespondence between the contact position used for a given stimulationwithin the cuff electrode and the quadrant of the visual field in whichthe volunteer drew the related phosphene. FIG. 4 shows a representationof the retinotopic arrangement of phosphenes according to the activatedcontact in self-sizing spiral cuff electrode 14. When a givenstimulating condition was applied to a given contact within the cuffelectrode, a phosphene is reported by the patient, provided that thestimulation threshold has been reached. The contact-quadrantrelationship which results from the optic nerve electrical activationwas consistent with the orderly arrangement of both quadrants andcontacts. This sample of 64 phosphenes collected near threshold alsoillustrates the broad distribution of relatively small phosphenes withinthe patient's visual field.

FIG. 5 shows a representation of the retinotopic organization of thepatient's optic nerve. The probable position of the 4 contacts, labeled0°, 90°, 180°, and 270°, around the optic nerve is indicated on theright, while on the left, the quadrant-contact relationship refers tothe position in the visual field of phosphenes elicited when stimulatingthrough a given contact. The retinotopy observed here for the rightoptic nerve is consistent with previous findings, although with avertical median slant less slightly than described. When compared toclinical data related to the retinotopy of the human optic nerve, when aslant of about 60° of the vertical meridian optic nerve projection(relative to the vertical axis) is reported, the observed resultsindicate a more limited inclination.

As expected, phosphene location depend on gaze direction. A steady gazeoriented sideways with respect to the fixation point during thestimulus, or a saccade ending just before the stimulus, resulting in aphosphene location consistently referring to gaze orientation at thetime of the electrical stimulation. Similarly, any gaze displacement,either after a time period or immediately after the stimulus, resultedin a phosphene location steadily referring again to gaze orientation atthe time of the stimulation (in this case, before the movement). Thus,stimulation resulted in phosphenes coded in spatial coordinates: i.e.,the algebraic substraction of gaze coordinates from retinal coordinatesfrozen at stimulation. Therefore, in order to secure an accuratemeasurement of phosphene attributes, the importance should be stressedto the patient of maintaining a fixed gaze during the presentation ofeach test stimulation.

In summary, the axons of retinal ganglion cells in this patient havebeen successfully activated by electrical stimuli applied to the opticnerve to evoke many distinct phosphenes over a large portion of thevisual field. Slight changes in the attributes of the phosphenes seem tooccur over time.

While the invention has been shown and described in terms of a preferredembodiment, it will be understood that this invention is not limited tothis particular embodiment and that many changes and modifications maybe made without departing from the true spirit and scope of theinvention as defined in the appended claims.

What is claimed is:
 1. A method for stimulating the optic nerve of amammal to induce visual perception in the eye, comprising the steps of:implanting a cuff electrode about the optic nerve; coupling said cuffelectrode to a signal generating controller; and transmitting a seriesof electrical signals from said controller to the optic nerve throughsaid cuff electrode in order to induce a visual response in the eye. 2.The method of claim 1, wherein said cuff electrode comprises a spiralelectrode.
 3. The method of claim 2, wherein said electrode isself-sizing.
 4. The method of claim 1, wherein said series of electricalsignals comprises a bipolar pulse train.
 5. The method of claim 1,herein said series of electrical signals comprises pulses having acharge density below 150 μC/(cm² phase).
 6. The method of claim 1,wherein said series of electrical signals comprises single pulses. 7.The method of claim 1, whereby said visual response comprises thegeneration of phosphenes within the visual field of the eye.
 8. A devicefor stimulating the optic nerve of a visually impaired patient,comprising: a cuff electrode, capable of being positioned about and incontact with the optic nerve of a visually impaired patient; and controlmeans, electrically coupled to said cuff electrode, for transmittingelectrical signals to the optic nerve through said electrode to inducevisual perception in said patient.
 9. The device of claim 8, whereinsaid cuff electrode comprises a spiral cuff electrode.
 10. The device ofclaim 9, wherein said electrode is self-sizing.
 11. The device of claim8, wherein said control means generates an electrical pulse train. 12.The device of claim 8, wherein said cuff electrode includes fourcontacts.
 13. The device of claim 8, whereby said electrical signalsgenerate phosphenes within the patient's visual field.
 14. The device ofclaim 8, wherein said electrical signals from said control meansstimulate the axons of the retinal ganglion cells of said patient.